The present invention relates generally to automated spray painting or coating equipment and, in particular, to an apparatus for controlling a paint canister filling operation.
In spray painting of various types of products, such as automobile bodies or automobile body panels, automated machinery has been developed to spray a succession of products in a continuous and rapid manner. One type of prior art spray painting device includes a canister that is adapted to be releasably attached to a robot arm. When the canister is empty, or when a change in paint color is required, the robot moves the canister to a docking station where the canister is exchanged for a full canister. While the robot is using paint from the full canister, the empty canister is being filled for the next job at the docking station.
The canisters include a piston slidably disposed therein whereby the position of the piston defines a desired volume required for xe2x80x9cfillingxe2x80x9d the paint canister. A first step in the filling operation is to extend the piston so that no paint remains in the canister. A supply valve is opened and paint under pressure from a paint supply forces the paint into the canister, which paint displaces the piston within the canister. One prior art method of determining the amount of paint in the canister is to monitor the flow of paint to the canister with a flow monitoring instrument. Another method is to monitor the position of the piston with a sensor, such as a potentiometer, which provides an analog signal to a paint supply control. When the control detects that the piston has moved to the position that represents the desired volume of paint in the canister, the supply valve is closed and the paint filling operation is complete. The filled canister is now ready to be exchanged when the robot returns to the docking station. Alternatively, an encoder, rather than the potentiometer, can be used to detect when the piston is at the desired position.
The sensor, whether the potentiometer or the encoder, however is a potential source for electrical and/or mechanical failure. In addition, the sensor must be made intrinsically safe or explosion proof as it must operate in hazardous environments, such as a paint booth or the like.
It is desirable, therefore, to provide a method and an apparatus for monitoring a paint filling operation that reduces the risk of failure and does not require intrinsically safe or explosion proof construction in order for the apparatus to operate.
The present invention concerns a method and an apparatus for monitoring a paint canister filling operation. The apparatus includes a paint canister having a paint receiving interior with a piston slidably movable therein. A position of the piston in the canister interior determines a selected amount of paint to be received by the canister. The canister interior is connected to a paint supply through a supply valve. An actuator is connected to the piston for applying a force to the piston, which tends to move the piston in the canister interior. A sensor is connected to the actuator for sensing a force applied by the actuator to the piston. A control means is connected to the supply valve, the actuator and the sensor. The control means operates the actuator to move the piston to and maintain the piston at a predetermined position in the canister interior and opens the supply valve to cause paint to flow into the canister interior. The control means is responsive to a signal generated by the sensor representing a force applied by the actuator to maintain the piston at the predetermined position to close the supply valve when the force applied indicates a selected amount of the paint has been received by the canister. Preferably, the actuator is an electric motor and the sensor senses a value of torque generated by the electric motor representing said force applied to the piston.
The method for controlling a volume of paint in a canister during a filling operation according to the present invention, the canister including a piston movable in an interior of the canister, comprises the steps of: applying a force to a piston to move the piston to a predetermined required position in an interior of a canister corresponding to a total volume of paint required for a painting operation; supplying paint to the canister interior; applying a force to the actuator to maintain the piston in the required position; comparing the force being applied to the piston to a set point force representing the force required to maintain the piston at the required position; and stopping the supply of paint to the canister interior when the force being applied to the piston exceeds the set point force. The method can include, prior to performing said step a., moving the piston to an extended position to expel any paint in the canister interior, determining the total volume of paint required and calculating the required position for the piston.
The present invention advantageously provides efficiency to the paint filling process by determining when sufficient material has entered the canister. In the prior art, the control software waits for predetermined time interval. When the time interval expires, the assumption would be that all the material has been loaded into the canister.
The present invention eliminates this assumption by providing confirmation that material has entered the canister by the motor torque achieving the specified threshold torque value. The present invention also advantageously provides a means for air contamination detection by monitoring the rate of change of torque. Paint contaminated with air will be more compressible and, therefore, will have a slower increase of observed torque.